Various process have been developed for treating wastewaters to remove suspended solids (i.e, suspended particulate matter, as distinct from dissolved solids such as salt in brine) from the water. Such processes are normally performed so that the treated water can be discharged into a river or other large body of water.
One such process involves the use of chemical flocculating agents. As used herein, the term "flocculant" refers to a chemical agent which is added to a liquid solution (such as a slurry, pulp, or sludge) for the purpose of causing particulate solids that are suspended in the liquid to form chemically bonded aggregates. Flocculants are also called coagulating agents or coagulants by some people. The particles formed by a flocculation reaction are referred to herein as floccules, aggregates, or floc. Since they are enlarged compared to the non-aggregated suspended particles, they settle more quickly in a settling tank, or they can be removed more easily using processes such as centrifugation or vacuum filtration.
One class of flocculents, developed several decades ago, includes certain types of metallic salts such as ferric chloride, ferrous sulfate, aluminum sulfate, etc. Those flocculants hydrolyze and/or ionize suspended solids, thereby causing those solids to react with each other and form aggregates.
Another class of flocculants developed more recently includes long-chain molecules with reactive side groups, such as polyacrylamide. The side-groups react with the solid particles suspended in solution, causing the particles to aggregate. Such flocculants are commercially available, or they can be synthesized using known techniques to have nearly any desired molecular weight, side group type and density, and ionic charge.
Another class of wastewater treatment processes involves the use of magnetic fields. If a magnetic field is applied to an aqueous solution containing particles which are magnetic, it will attract the magnetic particles, causing them to settle more quickly.
Those two processes--chemical flocculation and magnetic attraction--have been combined in various ways to create processes for removing both magnetic and non-magnetic particles from water. For example, U.S. Pat. Nos. 2,232,294 through '296 (Urbain and Stemins, 1941) teach the addition of magnetic powders and flocculants to wastewater, to form magnetic aggregates which are then subjected to magnetic fields to increase their settling rate. U.S. Pat. No. 3,142,638 (Blaisdell and Klaas, 1964) discloses the addition of "weighting agents" (including particulate iron ore) to polluted water along with cationic flocculants, and then subjecting the mixture to a magnetic field. U.S. Pat. No. 4,110,208 (Neal 1978) discloses the use of a flocculant which contains iron atoms to achieve the same effect. US Patent 4,193,866 (Slusarczuk and Brooks, 1980) teaches the use of ferrite powder, a flocculent, and an optional magnetic field to create a magnetic slurry, which can later be regenerated to recover the ferrite powder.
In addition, U.S. Pat. No. 3,536,198 (Bartnik et al, 1970) discloses a settling tank system with an inlet system which causes wastewater to flow through a magnetic unit before it reaches the quiet settling zone. Although the exposure to the magnetic field is brief (one or two seconds), it apparently aligns the particles in a way that promotes flocculation. That system was evaluated and discussed in an article by D. F. Beck and T. J. McBride in Industrial Waste, November 1969, pp. 5-9. In addition, German Offenlegungsschrift 26 16 734 (based on UK patent application No. 15756-75, by English Clays Ltd., 1976) discloses the use of a flocculating agent and a magnetic field to create aggregates, then passing the slurry through a magnetized filter.
In a different area, magnetic fields and flocculating agents have also been used in combination to remove magnetic impurities from kaolin clay; see U.S. Pat. No. 3,826,365 (Mercade, 1974).
Most of the prior art in the field of pollution control was developed to treat water which has relatively low concentrations of suspended solids. For example, the article by Peck and McBride involved water containing 600 to 15,000 mg of solids per liter of water; that is 1.5% or less solids by weight. By contrast, the wet scrubbers used to remove particles from blast furnace (BF) off-gases typically create sludges containing up to 25% solids, which would translate into more than 250,000 mg/l The sludges generated by basic oxygen furnaces (BOF) range up to 40% solids. The various types of furnaces and processes used for making steel, and the scrubbers which generate sludges with high solids concentrations, are described in various texts such as Making, Shaping, and Treating Steel, 10th edition (1985), published by the Association of Iron and Steel Engineers (Pittsburgh, Pa.)
As used herein, materials which are "generated during steelmaking" includes materials generated during pollution control operations, iron ore mining, and other peripheral operations which are related to the process of making steel. For convenience, the term "sludges" is used broadly herein; it includes slurries, pulps, sludges, and cakes (all are solutions, usually aqueous, that contain varying levels of suspended solids). In common usage, a slurry or pulp is a relatively thin mixture, a sludge is thicker, and a cake is solid or semi-solid, but those classifications are not exact and the term "sludge" is often used for convenience to represent any or all of them.
In efforts to dewater sludges generated during steelmaking, the solids concentration in the sludge represents only the starting point, and the goal is to increase the solids concentration to at least 70% or more, which will convert the sludge into a semi-solid cake having (1) a high metallic content so it can be recycled as feedstock for the steelmaking process, and (2) a low water content so it can be handled and possibly transported economically.
The disposal of semi-solid metallic wastes generated during steelmaking is a substantial problem. Roughly 40 to 50 pounds of dust are generated per tonne (metric ton; 2,200 lbs) of liquid steel. For an annual U.S. steel production of 100 million tons, roughly 2 million tonnes (dry weight) of dust are generated. Currently, most of this waste material is disposed of in landfills, or is held in lagoons. Due to the presence of zinc in those waste products (which results from the use of scrap metal as furnace feed material), most of those steelmaking wastes are classified as toxic and hazardous. Current landfill costs for hazardous wastes range up to $180 per tonne, and that cost is expected to increase dramatically in the future.
Some commercial recycling of steelmaking sludges is carried out, but it is generally limited to sludges having high zinc content, because the zinc makes the process economical, and steps must be taken to reduce the water content before transporting or recycling the waste. To the very limited extent that this is done at all, this dewatering is normally done by vacuum filtration, which can generate cakes having 70-80% solids by weight. Various types of filtering systems have been developed for such use; see, e.g., chapter 19 of Perry and Chilton, Chemical Engineers Handbook (McGraw Hill, 1973). However, sludge filtration suffers from various drawbacks. It is time-consuming; the filters clog up quickly, the sludges are difficult to handle and dewater, and it may be difficult to use filtration on a continuous basis. As a result of these and other drawbacks, filtering is not used widely to dewater steelmaking sludges.
The object of this invention is to provide an efficient and economic system for dewatering steelmaking sludges at relatively low cost (including equipment, manpower, operating, and energy requirements).